Why Do Leaves Change Color?

December 23, 2014
GSMNP Phenology DIA Image

October 31, 2014. Fall in Great Smoky Mountains National Park. This photo was taken from the Carlos Campbell Overlook on Newfound Gap Road (photo by NPS). 

Autumn in the eastern United States forests is an amazing color show that attracts sightseers world-wide. Factors including tree diversity, day length, and weather come into play for making this region a popular destination for viewing fall foliage. Favorite spots for “leaf peepers” to experience the autumn splendor include Shenandoah National Park, Blue Ridge Parkway, and Great Smoky Mountains National Park, which are all located along the Appalachian Mountain range.

The Appalachian Mountains are covered in a wide-variety of native deciduous tree species. The color of fall foliage depends on the tree species and the mixture of pigments in the leaf. There are three main pigments that give leaves their color and include chlorophyll, carotenoids, and anthocyanins. In spring and summer chlorophyll is the pigment that gives leaves their green color. At the end of summer, the decreased daylight and cooler temperatures, cause chlorophyll to break down, which expose the carotenoid (yellow, orange, and brown) and anthocyanin (red, purple, and crimson) pigments. Weather plays an important role in defining the richness of fall colors. A succession of warm, sunny fall days, followed by cool crisp, above freezing nights produce the most vibrant viewing conditions.

The Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation indices (MYD13Q1, MOD13Q1), can be used to monitor variations in global vegetation conditions, such as seasonal changes. These data products are 16-day composites with a spatial resolution of 250-meters, and include the Normalized Difference Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI).  Below are two 16-day MODIS EVI images from summer and fall over the Appalachian Mountains. These EVI images are classified using the greenness index ranging from 0 (no vegetation) to 1 (maximum cover) based on the amount of healthy green vegetation present. The summer image (left) contains lush green vegetation, as shown by the dark green colors. The fall image (right) shows decreased green vegetation due to the leaves changing color and dropping from the trees. Shenandoah National Park is located in the northeast corner of the image, and Great Smoky Mountains National Park is located in the southwest corner.

Use the slider bar to compare the MODIS EVI summer and fall image. 

EVI summer 2014 Phenology DIA EVI Fall 2014 Phenology

Left Image: MODIS 250-meter EVI image from summer (June 18 - July 3, 2014) over the Appalachian Mountains. This summer image is significantly greener, indicating healthy vegetation.

Right Image: MODIS 250-meter EVI image from fall (November 1 - 16, 2014) over the Appalachian Mountains. This fall image indicates a decrease in green vegetation due to leaves changing color and dropping from the trees.


National Park Service, Great Smoky Mountains, 2014, Fall Colors, accessed December 1, 2014 at


USDA Forest Service, Northeastern Area, 2014, Why Leaves Change Color, accessed November 17, 2014 at http://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm

USGS, Eastern Geographic Science Center, 2014, Phenology, accessed November 17, 2014 at


Palm, Carl E. Jr., 2014, Why Leaves Change Color. ESF, Environmental Information Series, accessed December 5, 2014 at http://www.esf.edu/pubprog/brochure/leaves/leaves.htm

Material written by: Kari Beckendorf1

Innovate!, Inc., contractor to the U.S. Geological Survey, Earth Resources Observation and Science (EROS) Center, Sioux Falls, South Dakota, USA. Work performed under USGS contract G10PC00044 for LP DAAC2.

2 LP DAAC Work performed under NASA contract NNG14HH33I.